The invention relates to a method of monitoring a monitored zone by means of an optoelectronic sensor, wherein transmitted light is transmitted in accordance with a structured light pattern in the direction of the monitored zone and is received from the monitored zone, the received light is converted into received signals and the received signals are evaluated with respect to the presence of an object, or with respect to a movement of an object, in the monitored zone. The invention further relates to a corresponding optoelectronic sensor.
Such a monitoring method and such a sensor serve, for example, for the securing of the monitored zone against an unauthorized intervention, that is, in that a switching off signal, or a warning signal, is generated if the presence of an object, or the movement of such an object, is detected in the monitored zone.
For this purpose, the monitored zone is actively illuminated with the transmitted light and the transmitted light received from the monitored zone is recorded by means of a reception device, for example by means of a CCD camera. The corresponding received signals of this reception device are compared with thresholds or with received signals which were determined and stored at an earlier time. A conclusion can be drawn from these on changes having taken place in the interim within the monitored zone.
With homogeneous objects, that is, objects which have only a low visual contrast themselves and in relation to the background, the problem can arise that an introduction of such objects into the monitored zone is not recognized without error. To avoid this problem, a structured light pattern is worked with, i.e. the transmitted light is transmitted in accordance with a light/dark pattern, and/or the transmitted light is recorded on the receiver side in accordance with such a light/dark pattern. When such a light pattern is used, for example with a triangulation arrangement of the transmission device and the reception device, the displacement of a homogeneous object can also be recognized. This procedure can, however, in turn bring about the disadvantage that the reception device cannot reliably be checked as to its functional ability (c.f. FIGS. 3 to 5).